The Immobilization Effect Of Green Synthesized Hydroxyapatite On Cadmium In Soil

Cadmium（Cd）is persistently toxic,non-degradable and highly mobile in the soil-plant system,and the transfer of Cd through the food chain poses a serious risk to human health.Therefore,the effective reduction of Cd toxicity in soils is of great importance for food safety and human health.Hydroxyapatite（HAP）is widely used in the remediation of Cd-contaminated soils due to its environmental friendliness and strong adsorption capacity for heavy metals.However,HAP is prone to agglomeration and high preparation cost,which limits its application in the remediation of Cd-contaminated soils.Therefore,the development of efficient and inexpensive HAP is a hot research topic in the field of Cd-contaminated soil remediation.To address the above problems,the thesis developed a green synthetic hydroxyapatite with straw extract as dispersant and ammonia evaporation waste as calcium source,and investigated the adsorption behaviour and mechanism of green synthetic hydroxyapatite on Cd in solution and its passivation behaviour on Cd in typical northern alkaline soil and southern acidic soil,providing technical support for the development of efficient and inexpensive HAP and its application in the remediation of Cd-contaminated soil.The main research of this paper is as follows:1.Green synthetic hydroxyapatite based on different straw extracts was prepared,and its adsorption behaviour and mechanism on Cd in solution were investigated.The results showed that the green synthetic hydroxyapatite has a large specific surface area and has a good adsorption effect on Cd with a maximum adsorption capacity of 222 mg g^-1.The kinetic data indicated that the adsorption of Cd on the green synthetic hydroxyapatite is an irreversible chemisorption process.XRD,TEM and other characterizations show that the mechanism of Cd adsorption by green synthetic hydroxyapatite is mainly a synergistic effect of ion exchange reaction and ligand complexation reaction.2.The passivation behaviour of Cd in a typical northern alkaline soil was investigated using green synthetic hydroxyapatite as a remediation agent.The results of the study revealed that the application of green synthetic hydroxyapatite significantly reduced the bioeffectiveness,phytoeffectiveness and bioavailability of Cd in soil by48.4%～57.8%,20.4%～28.6%and 12.6%～24.0%,respectively.In addition,the application of green synthetic hydroxyapatite improved the soil bacterial community and enhanced soil nutrient effectiveness.3.The passivation behaviour of green synthetic hydroxyapatite as a remediation agent was investigated for Cd in a typical southern acidic soil.The results revealed that all four HAPs promoted the growth of rice and reduced the translocation and accumulation of Cd in rice plants.the application of HAPs reduced the content of active state Cd in the soil.Using correlation analysis and principal component analysis,it was shown that the amount of HAPs applied was the main driver for inducing changes in soil p H,CEC,AP physicochemical properties and changes in the biological effectiveness of Cd in soil.The biological effectiveness of Cd in soil was reduced by26.10%～48.47%at 1%HAPs dosing.